The software is free, and you would never need to pay anything to get set up great system for yourself. In addition to that, they also offer a service that you can subscribe to that gives you some benefits. You can replicate all the functions on your own, I believe, but the service makes it extremely simple.
Home assistant does not, but Haier would rather you use their proprietary SmartAir2 app that vacuums up every tiny bit of personal data for resale, and could potentially turn into a subscription service later.
Mind you smartair2 has a 1.4 star rating on the play store if that tells you how well any of their software and devices work lol
IMO, it's also worth the yearly cost for voice integration with Google Nest/Home or similar devices. Both can be done without paying but it's so much easier and I know it'll be supported over time.
It may or may not be illegal, they're just throwing their weight around under the suggestion that it's illegal. Knowing well that a single dev working on a plugin in their freetime isn't likely to want to invest in legal proceedings.
Which is even more reason to do the above, to stick it to them.
It wouldn't be illegal if you hosted it in a country where there is no law against such a thing, and/or Haier does not have any presence which would grant them legal standing. Is anyone running a server farm in Sealand yet?
Why would it be illegal? A cease and desist is just a fancy letter saying do what we want or else we'll go crying to a judge. If the original dev hasn't done anything wrong (I'm not familiar with the project but I'd still say that's highly likely to be the case) then there's nothing to worry about. So make all the clones you want, and watch a dumbass company Streisand itself